This invention relates to tyre pressure monitoring apparatus for vehicles, and to a method of driving a set of initiators of such a system. It also relates to a control system for controlling a tyre pressure monitoring apparatus.
Tyre pressure monitoring (TPM) apparatus are known which comprise a plurality of tyre pressure monitoring transducers fixed to respective wheels of a vehicle. Each transducer includes a pressure sensor and a transmitter which is capable of transmitting an RF signal containing pressure information obtained by the sensor. These RF signals can be received and interpreted by a suitable receiver fitted to the vehicle. If the signal indicates a lower than usual pressure—which could indicate a leaking tyre—an error is flagged and the driver is alerted.
The transducers are often fitted within a tyre or as part of a valve stem. Each transducer may include a battery to provide a viable power source. Alternatively, if a battery-less transducer is used then each of the transducers would include a rechargeable power source such as an inductive loop with charging being achieved by applying a strong electromagnetic field of relatively low frequency LF to the loop. In either case the application of the LF magnetic field causes the transducer to wake up and send out a return signal in response. This field is produced by an initiator which should be fitted to the vehicle close to the transducer. If only one initiator is provided, all transducers would wake at the same time and send back pressure signals in response also at the same time. To prevent this happening each transducer is provided with its own initiator located close by on the vehicle. This allows identical transducers to be used whilst allowing individual transducers to be identified.
A problem can arise where the “correct” transducer on a tyre is in its farthest position from its corresponding initiator whilst a different transducer is in a more favourable position. For example, the correct transducer could be located at the bottom of a wheel, shielded from its initiator by a steel wheel rim whilst the wheel is also turned away on full lock. In prior art TPM systems, all initiators always send out signals strong enough to deal with such a worse case situation and ensure a response is always received. In this case, false returns may be received which come from other transducers on the vehicle in a more favourable position.